1. Field of the Invention
The present invention relates to electronic circuit devices, and particularly relates to an electronic circuit device capable of mounting electronic components at high density.
2. Description of the Related Art
In the field of mobile terminals and information appliances, electronic circuit devices composed of multilayer substrates capable of achieving higher packing densities have been used, as disclosed in PCT Japanese Translation Patent Publication No. 7-107954. The demand on such electronic circuit devices for size and weight reductions and higher functions has been increasing in recent years.
According to the demand, flexible cables and substrates, which can be bended, are often used to connect the above multilayer substrates.
FIG. 1 shows a top view (upper) and a longitudinal sectional view (lower) of an example of a rigid substrate and a flexible cable connected thereto with a connector.
In FIG. 1, a rigid substrate 1 includes three laminated insulating boards, such as glass-epoxy boards, having an electrical circuit made of copper foil. A flexible cable 2 is made of a polyimide film instead of an insulating board. The flexible cable 2 is used to connect the rigid substrate 1 to, for example, an external interface or another substrate (not shown).
The flexible cable 2 is connected to a right end region 1b of the top surface 1a of the rigid substrate 1 with a connector 3. That is, the region 1b of the top surface la of the rigid substrate 1 is assigned to the connector 3 to connect the flexible cable 2 to the rigid substrate 1. As a result, other electronic components such as semiconductor chips and resistors cannot be mounted in the right end region 1b. The rigid substrate 1 therefore has difficulty in providing electronic circuit devices with higher densities. Accordingly, a rigid/flexible substrate and a multilayer flexible substrate as shown in FIGS. 2A and 2B are used to eliminate the need for the connector 3.
FIG. 2A shows a top view (upper) and a longitudinal sectional view (lower) of an example of a rigid/flexible substrate. FIG. 2B shows a top view (upper) and a longitudinal sectional view (lower) of an example of a multilayer flexible substrate.
In FIG. 2A, a rigid/flexible substrate 11 includes two rigid boards 21-1 and 21-2 laminated with a flexible board 22 disposed therebetween. The flexible board 22 is longer than the rigid boards 21-1 and 21-2 by the length of a cable part (connection part) 22a for connection to another substrate. Unlike the flexible cable 2, the rigid/flexible substrate 11 has no connector so that electronic components can be mounted on the overall top surface 21a of the rigid board 21-1.
In FIG. 2B, a multilayer flexible substrate 12 includes two flexible boards 31-1 and 31-2 laminated with another flexible board 32 disposed therebetween. The flexible board 32 is longer than the flexible boards 31-1 and 31-2 by the length of a cable part (connection part) 32a for connection to another substrate. That is, the multilayer flexible substrate 12 differs from the rigid/flexible substrate 11 only in that the flexible boards 31-1 and 31-2 are used instead of the rigid boards 21-1 and 21-2. Accordingly, unlike the flexible cable 2, the multilayer flexible substrate 12 has no connector so that electronic components can be mounted on the overall top surface 31a of the flexible board 31-1.
There is another approach to eliminating the use of a connector. FIG. 3 shows a top view (upper) and a longitudinal sectional view (lower) of an example of a rigid substrate.
In FIG. 3, a flexible cable 42 is partially bonded directly to a right end region 41b of the top surface 41a of a three-layer rigid substrate 41 with, for example, solder or an anisotropic conductive film. The direct bonding of the flexible cable 42 to the rigid substrate 41 is used as one of the approaches to eliminating the use of a connector.